Modern Plastics Handbook 2011 Part 16 pdf

First, the majority of material in landfills was, in fact,biodegradable, consisting of paper, food waste, and yard waste.Second, conditions in modern landfills, designed to keep material

resin and fiber companies, and some is used by United Recycling, asubsidiary of Environmental Technologies USA, to make “grey felt”which is used for padding installed under commercial floor coverings,

as well as for sound insulation in automobiles.145

Collins & Aikman Floorcoverings, of Dalton, Ga., uses vinyl-backedcarpet to make solid commingled plastic products such as car stopsand highway sound-wall barriers They are now using up to 75%reclaimed carpet materials to make a nylon-reinforced backing for newcarpet for modular tile products.143

DuPont is investigating the use of ammonolysis to depolymerizemixed nylon 6 and nylon 6/6 from used carpet The company is alsousing reclaimed fiberized material from nylon carpet to make nylonbuilding products for use in wet environments such as kitchens andbathrooms.143

Researchers in Georgia are investigating an unconventional use ofcarpet fibers, incorporating them into the surface of unpaved roads toimprove road performance.146

To simplify the task of carpet material identification, the Carpet andRug Institute has developed a seven-part universal coding systemwhere a code on the carpet backing can be used to describe the com-ponents of the carpet, including facing, backing, adhesive, and fillers

As of 1997, it was estimated that 85% of the carpet now being made inthe United States uses this code However, the average 10-year lifespan of carpet means that for the next several years, most carpetentering the recycling stream will not be so labeled.143 Thus, as forautomobile parts, equipment for identifying carpet materials will con-tinue to be needed

12.4.16 Other plastics

While the major types of plastics recycling have been addressed, thereare a variety of other types of plastics recycling going on, often on asmall-scale or experimental basis For example, Arco ChemicalCompany has a process for recycling glass-reinforced styrene maleicanhydride from industrial scrap.147The University of Nottingham has

a project for developing recycling techniques for thermoset materials,including polyesters, vinyl esters, epoxies, phenolics, and amino resinsalong with glass and carbon-fiber reinforced resins.148The FraunhoferInstitute in Teltow is developing a process for recycling thermosetsusing an amine-based reagent in a one-step process which requires lit-tle added heat The process is said to be applicable to almost all ther-mosets.149 Imperial Chemical Industries plc and Mitsubishi RayonCompany Ltd are developing technology for recycling of acrylics bychemical depolymerization and repolymerization.150 The introduction

of polyethylene naphthalate (PEN) in U.S packaging markets wasdelayed by the perceived need to develop processes for automatic sep-aration of PEN from PET as well as technology for recycling of PEN.Other examples could also be cited The field of plastics recycling isconstantly evolving, in response to changing demands and opportuni-ties, as well as the emergence of new resins and new applications.

12.5 Overview of Plastics Degradation

Until the early 1970s, attention to plastics degradation, includingbiodegradation, was focused primarily on ensuring that the plasticmaterials being used were resistant to such degradation so that theycould maintain their usability Biodegradation and other types ofdegradation, such as photodegradation, were not always clearly differ-entiated Further, the extent of degradation was frequently measuredbased on loss of useful properties, such as tensile strength, rather than

on chemical changes in the polymer structure

In the mid-1980s, when concerns about solid waste disposal wereincreasing rapidly, there was again a flurry of interest in biodegrada-tion, stemming from a perception that disposal problems could be alle-viated substantially if we stopped filling up our landfills withnonbiodegradable plastics and instead switched to biodegradablematerials As information increased about both the composition andthe behavior of solid waste in landfills, it became clear that this was amisperception First, the majority of material in landfills was, in fact,biodegradable, consisting of paper, food waste, and yard waste.Second, conditions in modern landfills, designed to keep materials dry

to reduce problems with groundwater contamination, were not ducive to rapid biodegradation Pictures of grass clippings, vegetables,and hot dogs, still recognizable after 10 to 20 years in a landfill, rein-forced this reality, as did the statement by landfill researchers such asWilliam Rathje of the University of Arizona that landfill waste wasoften dated simply by reading the dates on the still-legible newspaperscontained in the garbage Additionally, it became clear that the “out ofsight, out of mind” approach to plastics degradation could not be justi-fied In other words, mechanical disintegration of a plastic productinto plastic dust was not equivalent to chemical breakdown and return

con-of the carbon and other elements to global cycles

Nonetheless, during the time when biodegradability was perceived

as a potent selling attribute for products such as merchandise sacksand garbage bags, a number of products were introduced which werecomposed of a mixture of starch, usually about 6% by weight, and low-density polyethylene Manufacturers of these materials claimed theywere biodegradable, based on the fact that the starch component

would be consumed by microorganisms if the materials were buried insoil and the argument that the then-fragmented plastic would also bebiodegraded Some manufacturers added pro-oxidants to enhance thefurther degradation of the materials The discovery that even materi-als such as food wastes often biodegraded very slowly under landfillconditions cast doubt on the claims, since even if the materials werebiodegradable, it was not clear that this degradability would serve anyuseful purpose Further, evidence suggested that the mere increase inexposed surface did not materially increase the biodegradability of thepolyethylene remnants The value of pro-oxidants in the anaerobicenvironment of a landfill was also questionable As a result, some envi-ronmental organizations began calling for boycotts of these products.Finally, some manufacturers of these products were charged by somestate attorneys general with misleading consumers, under statutesrelated to fair trade practices, and ordered to pay fines and cease mak-ing such claims Ultimately, these products disappeared, having suc-ceeded primarily in giving biodegradable plastics a bad name.

Since that time, two important changes have occurred First, a ety of truly biodegradable plastics have been formulated, and theirusefulness in niche markets, such as where plastics are likely tobecome a litter problem, particularly in bodies of water, has been rec-ognized Secondly, the use of composting as a waste management prac-tice has grown dramatically Composting is designed and managed topromote rapid biodegradation, so biodegradable products have assets

vari-in this scheme that they do not have where landfill or vari-incvari-ineration arethe usual approach to solid waste disposal This, then, is the motiva-tion for an examination of biodegradable plastics

12.5.1 Definitions and tests

Biodegradability of a plastic means that living organisms can use theplastic as a food source, transforming its chemical structure within areasonable period of time In practice, the organisms which we rely on

to accomplish this task are microorganisms, and the transformation ofchemical structure results in conversion of most of the carbon in thepolymer to carbon dioxide, methane, or other small molecules, alongwith some incorporation of the carbon into the cell mass of the microor-ganisms as they grow and reproduce The time period involved is usu-ally several weeks to several months

Unfortunately, as indicated earlier, there has been abundant misuse

of the terms “biodegradable” and “biodegradability,” with considerableconfusion resulting One of the main points of confusion has been themisidentification of photodegradable polymers as biodegradable.Photodegradation refers to loss of physical properties induced by expo-

sure to light A significant part of polymer research in the decades sincethese materials were introduced into commerce has been in the deter-mination of ways to minimize loss in strength in polymers which areexposed to sunlight Virtually all polymers have some tendency to pho-todegrade, so outdoor use of many polymers depends on the inclusion

of appropriate stabilizers to extend the plastic’s lifetime On the otherhand, it is sometimes desirable to hasten this light-induced degrada-tion Appropriate modification of the polymer’s chemical structure orthe use of additives can accomplish this accelerated degradation Suchplastics are properly termed photodegradable, but have been misiden-tified in many instances as biodegradable Photodegradable plasticsare outside the scope of this chapter

Another point of confusion has been the amount of chemical changeneeded to classify a plastic as biodegradable Early work in this areafollowed the practice initiated by those who were seeking to conservethe performance attributes of plastics and who measured degradation

by loss of physical properties such as tensile strength If one is ested in using a plastic for some purpose, the end point is clearly thepoint at which the plastic no longer has useful properties In this con-text, defining degradation in terms of loss of strength is perfectly rea-sonable However, if one is interested in total decomposition of thepolymer, it is not reasonable, since relatively few bond cleavages in apolymer backbone can destroy the polymer’s strength, while still pre-serving most of its chemical structure Many of the early discussions ofbiodegradable polymers failed to clearly make this crucial distinction.Another point of confusion has been the “reasonable time” part ofthe definition As is also true for photodegradation, the time requiredfor biodegradation is a function of exposure conditions (as well as afunction of the extent of degradation defined as the end point) Time toreach the defined end point after disposal can be markedly different ifthe object degrades in sewage sludge, in a compost pile, or in a land-fill, even under the same climatic conditions To this variation, then,must be added differences in ambient temperatures, rainfall, etc.Faced with a proliferation of environmental claims about productsand packaging, several state attorneys general produced guidelines forenvironmental claims, culminating in the issuing of the Green Report

inter-II, and took legal action against companies which they saw as makingmisleading claims In 1992, the U.S Federal Trade Commission (FTC)issued 16 CFR Part 260, “Guides for the Use of EnvironmentalMarketing Claims,” which was modified in 1996 and 1998 It includesguidelines for the use of degradability claims.151

The FTC guidance on the use of degradable, biodegradable, and todegradable is that such claims can be made only if there is “compe-tent and reliable scientific evidence that the entire product or package

pho-will completely break down and return to nature, i.e., decompose intoelements found in nature within a reasonably short period of time aftercustomary disposal.” Unqualified claims about compostability can bemade only if “all materials in the product or package will break downinto, or otherwise become part of, usable compost (e.g., soil-conditioningmaterial, mulch) in a safe and timely manner in an appropriate com-posting program or facility, or in a home compost pile or device.”151There has also been activity in the area of definitions, environmentalclaims, and testing of such materials by standards-setting organizations

on both national and international levels The American Society forTesting and Materials has issued several standards relating to degrad-ability in a variety of environments For example, ASTM D5338-92,

“Standard Test Method for Determining Aerobic Biodegradation ofPlastic Materials Under Controlled Composting Conditions,” providesfor measuring evolution of carbon dioxide after inoculation with com-posting microorganisms The percent of biodegradation relative to a cel-lulose reference is reported.152In Germany, the Fraunhofer Institute forProcess Engineering and Packaging (IVV) formulated a standard in

1998 for the compostability of biodegradable plastics, Standard DIN V

54900.153 The European Committee for Standardization (CEN) has

“Requirements for Packaging Recoverable through Composting andBiodegradation—Test Scheme and Evaluation Criteria for the FinalAcceptance of Packaging” in draft form.154In Japan, the BiodegradablePlastics Society has developed several standards for testing biodegrad-ability in different environments.155

The Degradable Polymers Council of SPI adopted definitions ofbiodegradable and compostable for plastic collection bags in 1998.Their standard is that “biodegradable” and “compostable” bags

“should, at a minimum, satisfy ASTM D5338 and D6002 tests showingconversion to carbon dioxide at 60 percent for a single polymer and 90percent for other materials in 180 days or less, and leave no more than

10 percent of the original weight on a 3/8″ screen after 12 weeks.”156Some early studies of biodegradation of packaging materials whichused growth of microorganisms as a measure came to misleading con-clusions For example, PVC was incorrectly determined to bebiodegradable since it supported growth of microorganisms However,later studies showed that it was the plasticizers in PVC which werebeing metabolized, not the PVC itself Care must be taken to avoidsuch misleading assessments In particular, a limited amount ofdegradation in a short time cannot be extrapolated to a conclusion ofsubstantial degradation at a later time In addition to the problem ofadditives, some parts of the structure of a polymer may well be moreresistant to degradation than other parts For example, crystallineregions in a semicrystalline polymer will be more resistant to degra-

dation than amorphous regions Also, chain ends are typically lessresistant than middle regions.

12.5.2 Effects of environment/exposure

conditions

As was mentioned earlier, rates of biodegradation are very sensitive toenvironmental conditions One result of this sensitivity is that degra-dation rates in modern municipal solid waste landfills tend to be quiteslow A key factor is the amount of water When landfills were found tooften be the source of pollutants entering groundwater systems, regu-lations were tightened to ensure that landfills would not generate sub-stantial amounts of liquid effluent which could make their way intowater systems Liners at the bottom of landfills and caps on the topwere added to the requirements, and it was no longer possible to buildlandfills in some high-moisture areas The caps, in particular, aredesigned to create a barrier to the ingress of water In addition,leachate (the liquid effluent from a landfill) must be pumped out andtreated before it is discharged Thus, relatively little water enters amodern landfill, and what does enter is routinely removed, so the land-fill environment is relatively dry Microorganisms do not grow asrapidly in such environments as they do when moisture is plentiful Inaddition to the moisture factor, landfills within a few years becomelargely anaerobic Trapped oxygen is used up, and cannot be replacedrapidly enough to maintain aerobic conditions Therefore, the types oforganisms predominating in an oxygen-rich environment will not beidentical to those which predominate in an oxygen-poor environment,and growth rates in general will be lower In addition to differences inthe amounts and types of microorganisms, some change their metabo-lism in response to oxygen availability The end result is slower rates

of decay, and a change from generation of carbon dioxide to generation

of methane While the methane can be collected, concentrated, andused as an energy source, it is also a potentially explosive gas and anair pollutant A modern landfill will continue to generate methane for

a substantial length of time after it is closed and capped Slowbiodegradation occurring over many years results in sinking and set-tling of the landfill area as well as generation of methane In addition,the leachate from a landfill can contain a variety of undesirable chem-icals Therefore, biodegradation in a landfill environment can havenegative environmental consequences

If it is desired to speed up decomposition in a landfill environment,leachate recirculation can be used In such systems, instead of pump-ing out, treating, and discarding any liquid effluent which reaches thetop of the landfill liner, the liquid which is pumped out is reintroduced

into the top of the landfill, creating an environment much richer inwater than is otherwise the case In such circumstances, biodegrada-tion will occur more rapidly, and the landfill will reach a stable condi-tion in a much shorter time The U.S EPA has been reluctant topermit such schemes, however, because of the significantly greaterpotential for groundwater contamination which also results.

In contrast to landfills, compost operations, which will be addressedfurther in Sec 12.5.3, are designed to ensure rapid biodegradation ofsusceptible materials In composting, a water and oxygen-rich envi-ronment is maintained, along with a suitable inoculation of microor-ganisms to start the decay process The time required for production

of usable compost varies, but is most often less than a year The posting operation can use open piles or windrows or closed containers

com-It can run on a very large scale as either a municipal or privatelyowned operation, or it can be a small compost pile in someone’s back-yard It can be an open-air facility, contained inside a building, oroperate in closed containers It can contain yard waste only, source-separated organics, or mixed municipal solid waste Composting ofyard waste has grown rapidly in the United States over the lastdecade, significantly influenced by legislation in many states whichprohibits the landfilling of yard waste In Europe, the scarcity of land-fill space led to adoption in many areas of systems for collectingsource-separated organics which are then composted In the UnitedStates, composting facilities which accept more than just yard wasteare still relatively rare

In addition to moisture levels and the amount of oxygen, ture plays an important role in determining the rate of biodegradation.Increases or decreases in temperature affect the growth rate andactivity levels of microorganisms, and hence the rate of biodegrada-tion Different types of organisms are at their best at different temper-atures Usually, the activity increases with increasing temperature,

tempera-as long tempera-as the temperatures do not get too hot Thus rates of dation in landfills are usually higher in warm climates than in colderones In a compost operation, degradation results in the evolution of

degra-a substdegra-antidegra-al degra-amount of hedegra-at, which rdegra-aises the temperdegra-ature cantly above ambient conditions A well-designed system will main-tain a sufficiently high temperature for long enough to kill pathogenswhich may be present, so the compost does not spread either disease

signifi-or weeds

Some biodegradable plastics may enter a different waste stream—liquid waste rather than solid waste Sewage treatment facilities, likecomposting operations, are designed to hasten biodegradation of thecollected wastes For plastic products or packages which may end up

in sewage systems, biodegradation is a significant asset Rates of

degradation in sewage are generally different than rates of tion in compost Holding times are also different; a product whichdegrades reasonably well in composting may not degrade fast enough

degrada-in sewage treatment facilities to avoid causdegrada-ing problems

We can conclude that in situations where landfill is the predominantdisposal option, biodegradable plastics generally offer no significantadvantage Rates of degradation are slow, and the products of degra-dation, while they may include generation of methane to be used asfuel, are largely undesirable Similarly, where incineration, with orwithout energy recovery, is practiced, biodegradable plastics are notadvantageous On the other hand, biodegradable plastics can play twotypes of roles in compost operations First, if composting of a mixedorganic stream is taking place, these materials will degrade along withthe paper, food waste, and other biodegradable components Second,even if yard waste is the only material composted, biodegradable plas-tics can be used as the bag in which compostable materials are col-lected Biodegradable plastics can also be advantageous for products orpackages which are likely to be disposed of in sewage

All the previous discussion is based on products or packages ing a regulated waste stream Products and packages can also be lit-tered or illegally dumped In such cases, if the item is not degradable,

enter-it may stay in the environment for a very long time In some cases, thispresents no problem The steel may slowly rust away at the bottom ofthe lake, the plastic bag may first be buried in leaf litter, and theneventually be covered up with soil Too often, however, the object is lessinnocuous The beverage ring connector may entrap a duck The float-ing plastic bag may be swallowed by a sea turtle who thinks it is a jel-lyfish A skunk may get its head caught in the yogurt container Theseencounters with wildlife can result in injury or death of the animal,either through entrapment in or ingestion of plastic items The prob-lem appears to be particularly acute when the plastics reach bodies ofwater, either by being thrown into the water, or being carried to thewater by runoff or storm sewer overflow Thus there is also a potentiallyvaluable role for degradable plastics in items which are frequentlylittered

An additional application for biodegradable plastics is in ships.Annex V of the International Convention for the Prevention ofPollution from Ships (MARPOL) prohibits ships from disposing ofplastics into the ocean While navies are exempt from the treaty, manynations, including the United States, have committed themselves torequire their military forces to abide by the treaty Ships of all sizesand types often have difficulty in appropriately storing or disposing ofwastes The problem is particularly acute for ships carrying largenumbers of people, ships which spend long periods at sea, or ships

operating in areas where ports are not equipped to off-load and dispose

of the refuse An aircraft carrier, for example, has about 6000crewmembers and serves 18,000 meals a day, generating substantialamounts of trash Nuclear submarines spend months continuously atsea Development of biodegradable plastics, which in some areas could

be disposed of in the ocean along with food wastes, could be a erable asset.157

consid-12.5.3 Composting

Composting of municipal solid waste in the United States is still in itsinfancy According to BioCyle, 18 such facilities are currently in opera-tion, and two are under construction.158Some facilities process a mixedwaste stream and others process source-separated organics (that is, thegenerators of the waste separate the compostable organics from thenoncompostable wastes) Generally, the source-separated organicsstream contains paper and food waste, and does not include any plas-tics Some institutional composting of source-separated organics, such

as of waste from fast-food restaurants, has included biodegradableplastics used for food containers and cutlery The mixed-waste com-posting facilities do include plastics among the materials collected TheEPA counted 14 such mixed waste composting facilities in 1996, han-dling a total of about 900 tons/day.2In such systems, the waste is gen-erally processed to remove large items, ferrous metals, and sometimesother components before processing The compost produced in thesefacilities is generally higher in levels of contaminants, including unde-sirables such as heavy metals, than compost originating from source-separated streams At their current level of use combined with thecurrent level of nonyard-waste composting, biodegradable plastics playonly an insignificant role For biodegradable plastics to have a signifi-cant impact on waste management, a higher level of use and a largegrowth in composting programs would be required If source-separation

is part of the system, significant consumer education efforts would also

be needed to get people to divert only the right kinds of plastics to thecompost stream

Composting of yard waste is more prevalent in the United Statesthan composting of other waste streams In 1996, the U.S EPA reported

3260 yard waste composting programs, handling approximately 25,500tons/day.2 Some facilities collect yard waste only in paper bags, whichcan be composted along with the yard waste, but which can cause prob-lems if they become wet and consequently weaken Some facilities col-lect leaves in loose form using a vacuum system Some facilities permitplastic bags, but remove the yard waste from the bags before compost-ing This can add significantly to the cost of composting A number of

facilities have tried using biodegradable plastic bags, with mixedresults The variability in types of bags tested along with variability incomposting conditions make it difficult to assess the suitability ofbiodegradable bags without field testing For example, seven brands ofbiodegradable bags were field tested in Northville, Mich Four bagswere found to leave no visible residues after that facility’s standard 8-week composting time, while three bags were readily visible and iden-tifiable Investigators pointed out that in differently designed facilities,results could have been quite different.159In Michigan, legislation wasintroduced in 1998 and passed by the state House to ban the use ofmost plastic bags for yard waste collection The bill originally bannedthe use of any type of plastic bag, but was modified to exempt bagswhich are certified by the state as meeting compostability standards.The bill is now under consideration in the state Senate.154A number ofyard waste composting operations, in Michigan and elsewhere, alreadyprohibit the use of plastic bags, based on difficulties they have encoun-tered in the past.

Minnesota is the state with the largest number of municipal posting facilities in operation More than 400 composting facilities werestarted as a result of state laws passed in 1978 and 1980 that requireddevelopment of alternatives to landfills Most process only yardwaste.160 Eight of the top 10 solid waste management companies inNorth America had an involvement in composting programs by 1996.Many operated compost programs on the same site as landfills.161

com-In 1998, Portland, Oreg., became the first major U.S city to

consid-er mandatory food waste composting The proposal recommended tothe City Council would require participation by some grocery stores,restaurants, and food processors, but not by individual residents Thecity says commercial food waste represents the largest single material

in the current solid waste stream, and so it is important to target inattempts to meet the city’s 60% recovery goal.162

Composting is much more prevalent in some countries than in theUnited States In Canada, for example, increasing composting was animportant part in successfully reducing waste disposal to 50% of the

1988 amount Although more prevalent (proportionally) than in theUnited States, facilities handling more than yard waste are relativelynew, and operators are still learning about the most effective ways tostructure the systems.163Montreal collects compostable food residualsand yard trimmings from 17,000 households in 25-L brown plasticbins, with disposable plastic liners, and also provides a 7-L containerfor kitchen discards Compostables are collected at curbside weekly.164Composting has a significantly longer history in Europe, where the first composting plants for mixed municipal solid waste date to the1970s, and it is estimated that more than 10 million tons of compost

are produced per year.165,166 In 1998, the European Union adopted a

“Common Position,” passed by the Environmental Council in March

1999, intended to reduce the amount of biodegradable material ing landfills It requires reduction of landfilling of biodegradablemunicipal solid waste to 75% of 1995 levels by 2006, with further tar-gets culminating in reduction to 35% of 1995 levels by 2016 EuropeanUnion members have 2 years to include these requirements in nation-

enter-al laws One result is expected to be a significant growth in ing Despite this history, the overall rate of composting of municipalwaste in western Europe is estimated at only 4% Many of the originalcompost facilities have closed due to lack of markets because of poorquality of the compost, including high contents of heavy metals and ofnoncompostables such as glass, plastic, and metal One consequencewas the development in several countries of organizations which cer-tify the type and quality of compost generated.165,166Another is a shiftfrom facilities treating mixed MSW to those treating source-separatedorganics Germany is reported to have more than 400 such facilities,with France having about 200, Italy 60, and Spain 30 These countries,along with Denmark and The Netherlands, are regarded as leaders incomposting of source-separated organics, and generally produce high-quality compost.167 Composting rates for household organics are sig-nificantly higher than the overall compost rate in the European Union,although they vary considerably among countries In the Netherlands,

compost-it is estimated that 90% of household organics are recovered throughcomposting, Germany recovers about 45%, and Britain only 6%,despite a five-fold increase in composting between 1993 and 1997.168The Netherlands achieves its high rate of composting by providingsource-separated collection of residential organics, including yard, gar-den, and kitchen wastes, in virtually all municipalities, on at least a bi-weekly basis As of January 1994, municipalities are mandated toprovide this service, and residential organics are banned from landfill It

is estimated that such organics make up as much as 60% of residentialwaste in some communities, although the average is in the 40 to 50%range Sixty to 75% is estimated to be garden waste, with the remainderfood waste Collection is typically provided in rolling carts collected bysemi-automated compactors in single-family housing, with organics col-lection alternating on a weekly or biweekly schedule with collection ofmixed waste Residents in most municipalities are required to wheel thecarts to the corner for collection.169

12.6 Natural Biodegradable Polymers

One way to obtain biodegradable plastics is to use natural polymers,that is, those formed by living organisms There is, in nature, a general

rule that what is formed by organisms can be decomposed by otherorganisms as part of the natural cycling of carbon in our environment.Thus, even very large polymer molecules such as lignin and cellulosecan be decomposed by a variety of microorganisms.

While cellulose and cellophane are not plastics, there are plasticssuch as cellulose acetate, cellulose butyrate, cellulose acetate-butyrate, etc., which are derived from cellulose Few studies of thedegradability of these materials have been carried out, and they arenot used in large quantities A general rule is that the greater theextent of chemical substitution in the molecular structure, the slowerthe degradation rate of the material Little attention has been given tothese materials in efforts to develop biodegradable plastics Slowdegradation rates, high cost, and processes which generate some nox-ious discharges are likely the reason There are other naturalbiodegradable polymers which offer more promise

12.6.1 Bacterial polyesters

One of the early truly biodegradable polymers was tyrate/valerate (PHBV) This is a member of the polyester familywhich is produced by certain types of bacteria when they have a dietwhich is carbon-rich but poor in some essential nutrient Under theseconditions, they produce polyhydroxybutyrate (PHB) as a food store to

polyhydroxybu-be called upon when carbon sources are less available With lation of the diet, the bacteria can be induced to form a copolymer,PHBV, which has more useful properties than PHB

manipu-While the polyesters grown in bacteria have been known for over 60years, and their use had been investigated by some companies such as

W R Grace & Company during the 1960s, modern development of teria-grown polyesters began with Imperial Chemical Industries (ICI)

bac-in Britabac-in, around 1978.170,171ICI used the bacterium Alcaligenes phus grown on glucose to produce poly(3-hydroxybutyrate) (PHB) in

eutro-80% yield, based on dry weight of the fermentation broth While PHBwas comparable to polypropylene homopolymer in melt point and ten-sile strength, and could be used to form small items such as golf tees byextrusion, it had a high glass-transition temperature and low extension

at break In 1991, ICI directed efforts toward purifying the polymer andadding plasticizers and fillers to alter its properties Costs were alreadyrecognized as a problem, since the cheapest glucose source, corn, wasalmost 3 times as expensive as raw materials derived from oil or nat-ural gas.171 By 1987, ICI was manufacturing copolymers as well asPHB, and was marketing the resins on a limited scale in Europe, most-

ly in Germany, under the trade name Biopol The resins could beprocessed into blown film, cast film, or calendered, and in trial runs,

had been made into bottles by both injection and extrusion blow ing Rates of degradation had been examined, with the finding thatanaerobic sewage provided the most rapid degradation, followed bywell-watered soil, seawater, sediments, aerobic sewage, cattle rumen,sea water, in vivo, and lastly with only a negligible degradation rate inmoist air.172 The polyester was described as a highly crystalline, stiffmaterial with a melt point of about 175°C and a glass transition tem-perature of about 0°C.173

mold-By 1987, ICI had focused on the random copolymer of butyrate andvalerate, PHBV, and had patented a process for making the material.The feedstock for the copolymer was glucose plus propionate.174Copolymerization provided significantly improved properties, includ-ing reduced stiffness and a broader processing window The homopoly-mer degrades at a temperature only 5 to 10° above its melting point,for example, while the copolymer has a much wider processing win-dow Marlborough Biopolymers, Ltd., an English subsidiary of ICI,produced the polymer in ton quantities for development of applications

in surgical devices, personal hygiene products, and packaging.Properties were described as intermediate between PP and PVC.Degradation rates ranged from several days for films in anaerobicsewage to 9 months for a bottle in soil.170By 1991, PHBV was beingproduced at about 25 tons/year, and was being used by Wella, aGerman company, for shampoo bottles The $15/lb price, when PP wasselling for 50¢/lb, was a significant deterrent to widespread use.174Thepromise was sufficient, however, that ICI announced an expansion ofcapacity to 300 tons/year, with plans for a commercial-size plant with

a 10,000-ton capacity in the next 3 to 5 years.174

The general process for manufacturing bacteria-grown polyesters,specifically polyhydroxyalkanoates (PHAs), is to deplete a growing cul-ture of a nutrient, such as nitrogen, which it requires to grow, causinggrowth to stop Then one or more carbon sources, such as glucose, areadded to the fermenter The bacteria accumulate pellets of polyester,about 2 m in diameter and of irregular shape, within the cells Underappropriate conditions, up to 80% of the total cell mass, or 80 g/L, ofpolyester can be accumulated To harvest the polyester, the granulesare extracted from the cells, purified, and formed into pellets.Crystallinity of PHBV is typically 70% or more, and molecular weights

of about 600,000 are desired.174

By 1991, a large amount of research was going on at universitiesand other companies around the world in efforts to develop naturalbiodegradable polyester polymers and copolymers One of the earlyentries was btF mbH, a state-owned subsidiary of Petrochemie

Danubia, in Austria In 1991, they were using Alcaligenes latus to

pro-duce 20 tons of PHB per year for controlled-release drug-delivery

sys-tems, and planned to increase production to 1 ton/week One differencewith this strain of bacteria is that it produces PHB throughout itsgrowth phases, rather than only during nutrient deprivation.174Japan at this time had two entries in the biodegradable polyester

race The Seibu Gas Company Ltd used A eutrophus in very pure

car-bon dioxide made from synthetic natural gas to produce PHB TheResearch Laboratory of Resources Utilization at the Tokyo Institute of

Technology used A eutrophus to produce a copolymer P(3HB)-co(4HB),

or 3-hydroxybutyrate with 4-hydroxybutyrate.174

In the United States, the applied microbiology lab at the MassachusettsInstitute of Technology (MIT) was working on bioengineering of polyhy-droxyalkanoate (PHA) biopolymers using recombinant DNA Researchers

at James Madison University successfully cloned the genes for making

PHB in A eutrophus and transferred them to E coli However, they failed

to get PHBV from the modified E coli when it was fed the same types of sugars and other nutrients as A eutrophus Researchers at the University

of Massachusetts were working on developing novel bacteria-grown

aliphatic polyesters, and produced a biodegradable elastomer using P oleovorans Researchers at MIT were working on bioengineering PHA

polymers using recombinant DNA.174 In 1992, researchers at MichiganState University and James Madison University reported the transfer of

the three genes responsible for synthesis of PHB from A eutrophus to a

plant related to mustard, and succeeded in getting production of the tic in the plants, although very little plastic was produced, and growth ofthe plants was significantly reduced.175Further genetic modification sig-nificantly increased production of plastic.176Researchers at MIT received

plas-a pplas-atent for trplas-ansfer of PHBV genes to bplas-acteriplas-a plas-and crop plplas-ants.177Amongtheir accomplishments was production of cotton fibers containing a corewhich, rather than being hollow, contains PHB, although in amounts lessthan 1% Researchers at the Volcani Center of the Israel Ministry ofAgriculture have also produced these materials.178

In 1992, ICI, with its production capacity now at 600,000 lb/year inthe United Kingdom, found its first North American market in blow-molded bottles and injection-molded caps for hair-care products The

$8 to $10/pound price was expected to fall to $4/lb as plannedincreased capacity came on-line.179In 1993, ICI got out of the biopoly-mer field, transferring Biopol to its spin-off business unit, Zeneca.180In

1996, Zeneca’s Biopol unit was sold to Monsanto, which continued tomarket PHBV under the Biopol trade name In addition to using bac-teria, Monsanto investigated production of PHBV in plants, such assoybeans and cannola, using recombinant DNA.181

One high-profile use for Biopol is credit cards for Greenpeace, firstavailable in 1997, in Europe.182 Other uses include disposable cups,eating utensils, planters, fish nets, compost bags, and packaging for

beauty products, with applications continuing to be limited by the highprice of the resin compared to competitive materials.183A unique prod-uct, manufactured in Denmark, is a twin-blade razor with handle andreplaceable heads both made with PHA Interestingly, the packagingfor the razor is a combination of polystyrene and paper.184In late 1998,Canada Trust chose Biopol as the material for some of their creditcards at about the same time that Monsanto announced that it wouldstop both production of Biopol and research and development on it bythe end of the year.185,186

Metabolix, located in Cambridge, Mass., is producing sample tities of PHB and of other PHA plastics One member of this family,polyhydroxyoctanoate (PHO), is a rubbery elastomer, with an exten-sion at break of 380%, compared to only 5% for PHB It also has lowertensile strength than PHB.187

quan-Researchers at the Universiti Sains Malaysia have been working onproduction of PHB from palm oil, and have found a number of localmicroorganisms which can be used to do so Efforts include identifyingthe genes responsible for manufacture of the polymer.16

More than 100 PHA polymers are known to be produced by bacteria.Polymer properties are a function of chemical composition and of aver-age molecular weight Researchers at the Massachusetts Institute ofTechnology have been able to control the molecular weight of PHB by

transferring genes from A eutrophus to E coli, and inserting

addi-tional genes to produce a larger amount of PHA synthase, which is theenzyme which links the individual polymer units together.188

Researchers at the Japanese Institute of Physical and ChemicalResearch, Riken, in 1997 announced the development of a 3-hydroxybu-tane acid/3-hydroxyhexanoic acid copolymer, which has a higher densityand higher strength than other PHAs Its melting point was listed asabout 300°C.189

PHB and other PHAs can also be blended with other biodegradablepolymers In Japan, the Ministry of International Trade andIndustry’s Biological Industry Institute in 1990 announced the devel-opment of blends of PHB and polycaprolactone (PCL) The blends can

be processed with conventional equipment, and the ratio of the twopolymers determines the rate of degradation.190

Chemical synthesis of biodegradable PHAs is limited by the need tohave stereo-specific polymers to obtain desired performance proper-ties The cost of single-isomer raw materials is quite high, renderingthe polymers as expensive as those obtained by the biological route,

so this approach has not been utilized commercially.191 L-isomers ofPHB, which are not made in bacteria, can cause spontaneous abor-tions, while D-isomers are nontoxic and can be safely used in thebody.174 Conventional chemical processes cannot, to date, produce

pure D-isomers.

In contrast, biological systems produce polymers with all monomers

in the D configuration The general chemical structure of yalkanoates made by natural processes is -[O-CHR-CH2-CO]n- wherethe side-chain R is a straight chain, and contains between 1 and 11carbons The monomers from which natural PHAs are polymerizedare all 3-hydroxyalkanes A large range of bacteria types are known

polyhydrox-to synthesize PHAs Low molecular weight PHB has also been found

to naturally occur in plants and animals, including humans In PHAplastics, molecular weights range between 2  105 and 3  105.Within the bacterial cells, PHB is in an amorphous form However, itcrystallizes during purification processes, treatment with solvents,heat, or if all water is removed One possible explanation for this fact

is that the rate of crystallization is very slow, unless nucleatingagents are present, in which case it is very rapid Another hypothesis

is that water or lipids present in the cell act as plasticizer, preventingcrystallization.191

Several types of bacteria are capable of forming PHB or of ing PHA/PHB copolymers from a mixed feedstock containing appro-priate precursor molecules The amount of hydroxyvalerate (HV)depends on the feed and also on the species of bacteria which is used

produc-However, only the Pseudomonas species of bacteria can produce and

store polymers from monomers containing 8 or more carbons.191

A eutrophus, P oleovorans, and several other organisms are able to

form and store polyesters from monomers other than

3-hydroxyalka-nes or alkanoic acids under appropriate conditions P oleovorans can

use a much wider variety of substrates than microorganisms in the

Alcaligenes family In addition to homopolymers and copolymers, polymers have been created in this manner For example P acidovo- rans fed with 1-4-butanediol and pentanol creates a terpolymer of 3

ter-hydroxybutyrate (3HB), 4HB, and 3HV It is also possible to rate potentially reactive substituents in the R side chain, which might

incorpo-be used for later cross-linking reactions or some other type of tization to modify polymer properties.191

deriva-PHAs containing longer chain length monomers than PHBV tend to

be elastomeric rubbery materials Little is known about the properties

of polymers formed from unusual monomers which yield aromatic,branched, or substituted side chains.191

Commercial production of PHB and PHBV is carried out in batch tems as large as 200,000 L in capacity The medium used in the fer-menter in the production stage contains glucose, a desired amount ofphosphate to produce the required amount of biomass, and an excess ofall other nutrients The culture is then inoculated with microorganisms,which reproduce until phosphate is depleted, and then begin to store

sys-polymer inside the cells Glucose is fed to maintain the sys-polymer tion, if PHB is being produced If PHBV is desired, a mixture of glucoseand propionic acid is fed to the fermenter The proportion of HV units inthe random copolymer is controlled by adjusting the ratio of glucose andpropionic acid An early problem with this scheme was that the bacteriaconverted much of the propionate to acetate or to carbon dioxide Sincepropionate is more expensive, this increased costs A mutant form of thebacteria was identified which cannot metabolize propionate to acetate,thus improving conversion efficiency and lowering costs.191

produc-The first route used for harvesting the polymer from the bacterialcells involved the use of large quantities of solvent, about 20 times theamount of polymer recovered This large excess was needed because ofthe high viscosity of the solution, even when very dilute The mosteffective solvents were chlorinated alkanes Such heavy use of theseenvironmentally problematic solvents was undesirable, and eventually

an aqueous system was developed to wash the polymer free from celldebris The result is production of a white powder, which is subse-quently melted, extruded, and pelletized The aqueous effluent fromharvesting is suitable for conventional activated sludge treatmentbefore discharge.191

Some properties of PHB, PHBV copolyesters, and PHO are rized in Table 12.3 PHB is a highly crystalline material, forming aright-handed helix when crystalline or in chloroform solution PHBVcopolymers with HV content up to 30% have HB units in the crystallattice, and HV units are excluded At HV content above 30%, HV occu-pies the crystal lattice, and HB is excluded The copolymers have beenshown to be random.191

summa-A variety of microorganisms, including both bacteria and fungi, havebeen shown to degrade PHB and PHBV Degradation rates are fasterfor PHBV than for PHB in all types of soils In water, rates for thehomopolymer and copolymer are identical Degradation rates aremuch faster in salt water than in freshwater In simulated compostand landfill conditions, PHBV degradation has been shown to be faster

in anaerobic conditions than in aerobic ones.191

12.6.2 Starch-based plastics

As mentioned, some of the plastics which purported to be able in the late 1980s and early 1990s contained polyethylene blendedwith about 6% starch While the starch was biodegradable, there was

biodegrad-no convincing evidence presented that the polyethylene itself

biode-graded to any significant degree The term biodeterioration has been

coined to reflect what happens in these materials: they lose strengthand are fragmented, but the molecular structure is not much affected

This led some environmental groups, such as the EnvironmentalDefense Fund (EDF) and the Environmental Action Foundation(EAF), to call for a consumer boycott of degradable plastics.192In con-trast to these materials, starch-based plastics have been developedwhich are truly biodegradable Some contain nearly 100% starch, andothers are blends of starch with other biodegradable components Mostuse carefully controlled amounts of water as a plasticizer to convertthe starch into a thermoplastic, along with carefully controlled tem-perature and pressure.

An early entry into the all-starch biodegradable plastics was Lambert, which in 1990 announced the creation of the first biodegrad-able plastic from starch According to the company, either potato, corn,rice, or wheat starch could be used These materials were sold under thetrade name Novon, and were water-soluble as well as biodegradable.Novon contained about 70% branched starches and 30% linear starch,along with a glyceride as an internal lubricant Properties included ten-sile properties comparable to crystal polystyrene, optical properties likethose of polyethylene, and elongation at break of about 20%.193Productsincluded compostable bags and cutlery In 1993, after Warner-Lambertwas unable to sell the Novon business unit, it suspended operation.Then, in 1995, EcoStar International acquired the unit, and formedNovon International, which shortly thereafter was acquired as a whollyowned subsidiary of Churchill Technology, Inc.194 Novon customersincluded Doane Products Company of Joplin, Mo., which used Novon fordog bones, and Eco-Turf Inc in Chicago which used Novon forbiodegradable turf tacks for golf courses, landscaping, and agricul-ture.195In late 1996, Churchill Technology filed for bankruptcy protec-tion under Chapter 11, and announced the intent to reorganize withbiodegradable plastic as their main product The company had notreported a profit in the last 2 years, since its merger with EcostarInternational.196No current information about its status was found.StarchTech, Inc., of Golden Valley, Minn., sells biodegradable starch-

Warner-TABLE 12.3 Properties of PHB, PHBV, and PHO 174,187,191

Tensile Flexural Melting point, strength, modulus, O2 permeability,

based resins for injection molding and other manufacturing processes,with a claim that in volume applications they approach the cost ofstandard plastics Target markets are landscape products, disposablefood service items, golf tees, and some pet products Polysheaf resinsare based on wheat starch and intended for injection molding Thecompany also sells Re-NEW, Bio-Lapse, and Bio-Revert resins.197The Natick Research Development and Engineering Center(NRDEC) has investigated the use of starch-based blown film, amongother products, to help the U.S Navy comply with MARPOL require-ments (no discharge of plastics at sea).157

Researchers at the Central Tuber Crops Research Institute (CTCRI)

in India have developed a starch-based plastic film for use in nurserybags for seedlings The country has a massive forestation programunderway, which requires about 65,000 tonnes of nursery bags peryear The raw materials for the film are tapioca or corn starch, urea,and water, along with a coupling agent It can be formed on conven-tional blown film equipment.198

One of the most visible demonstrations of starch-based able plastics was at the 1998 Winter Olympics in Nagano, Japan,where the “Eco-Friendly Kentucky Fried Chicken” booth usedbiodegradable plastic bags, flatware, cups and coated paper products,all based on corn The effort was sponsored by the U.S Feed GrainsCouncil.199

biodegrad-Research on starch-based plastics has taken place in many tries around the world The Australian government funded a 1995research project on development of starch-based plastics from cornand wheat, using water and glycerine as a plasticizer.200 In Japan,the Biodegradable Plastics Society was formed in 1989, with 48 mem-ber companies located mainly in Japan By 1990, the membershiphad expanded to 69 companies, and included a significant number ofnon-Japanese members.201 In 1992, the U.S Bio/EnvironmentallyDegradable Polymer Society was formed, and had over 200 members

coun-by 1998.202

Several companies sell starch-based foam peanuts for loose fill aging as a substitute for expanded polystyrene One of the first wasAmerican Excelsior, which manufactured Eco-Foam loose fill, and thenexpanded their line to include sheet and laminated structures.Originally, the materials contained 95% starch and 5% polyvinyl alco-hol.203 Later, American Excelsior manufactured Eco-Foam materialscontaining over 99% corn starch from a special hybrid corn.204 Theseproducts, in addition to being biodegradable and water-soluble, alsohad the advantage of dissipating static charges much more effectivelythan expanded polystyrene The company also claims that, because ofthe purity and absence of odor of their materials, bugs and rodents are

pack-not attracted like they are to other natural cushioning materials.200Clean Green Packing, of Minneapolis, Minn., a subsidiary ofStarchTech, Inc., began marketing starch-based loose fill cushioningmaterials in about 1992.197,205 FP International sells Flo-Pak Bio 8loosefill, made from corn, wheat, or potato starch, along with poly-styrene loosefill, including several grades made with high percentages

of recycled PS.206

Starch-based materials have also been designed for applications whichconventionally use urethane-based foam-in-place packaging In 1996,Environmental Packaging L.P developed a device which propels a wheatstarch-based material into a container holding an item to be shipped.The foam pieces are molded together using water, which is sprayed light-

ly on the foam pieces as they enter the container Development of thematerial involved a collaboration of Enpac, which is a DuPont andConAgra Company joint venture, and the Norel Paper Co.207,208

Biodegradable plastics which contain a mixture of starch and otherbiodegradable polymers, either natural or synthetic, have also beendeveloped For example, Yuking Company in Korea in 1997 announcedthe development of a biodegradable plastic containing polycaprolac-tone and more than 40% starch The starch component was includedprimarily to lower the price.209Biotec, a German company, developed abiodegradable material made of starch combined with a biodegradableplastic for use in biodegradable garbage bags Fardem Belgium, whichmanufactured the bags, received the first European “OK Compost”label for garbage bags This label is certification that the product iscompletely compostable, leaving no plastic remnants after the compostprocess is completed.210 In the United States, BioPlastics, headquar-tered in Michigan, manufactured biodegradable yard waste bags man-ufactured from starch and polycaprolactone for a trial program testingtheir use in a yard waste composting operation.211The Mellita Group

in Europe sells biodegradable compost bags under the Swirl tradename The bags are made from potato starch and polycaprolactam,and are reported to have properties similar to polyethylene The bagsdegrade to carbon dioxide, water, and humus in about 40 days in com-post piles The bags sell for approximately 5¢ each, compared to 3¢each for comparable LDPE bags Melitta also markets resin, which itsold for about $2.40/lb in 1995.212

Novamont, a subsidiary of the Italian company Montedison, keted Mater-Bi biodegradable film for composting and waste disposal

mar-A typical resin contains 60% starch along with other materials

Mater-Bi resins containing starch and poly--caprolactone can be handled onconventional film blowing and sealing equipment for LDPE, withminor modifications.213 One U.S manufacturer of trash bags madewith this material is Biocorp USA, which sells them as

“Materbags.”214,215 The bags have been demonstrated as suitable forvermicomposting (using earthworms) in addition to standard compost-ing In vermicomposting, the biodegradation was essentially complete

in 20 to 22 days, compared to 20 to 35 days in standard compostingoperations.215Resins are available in both molding and film grades.216

A high-tech application for starch-based biodegradable materials is incomposites formed from starch polymers and bonelike ceramics formedical applications Mater-Bi plastics containing a cornstarch/ethyl-ene vinyl alcohol blend reinforced with hydroxylapatite particles pro-duced biodegradable materials with promise for some suchapplications.217 In 1996, Montedison sold the Novamont unit, whichcreates and markets Mater-Bi, to a group of commercial banks head-quartered in Italy In 1997, production capacity was doubled, andNovamont GmbH in Germany was started to expand the Europeanmarket Biocorp, Inc., of Redondo, Calif., is the exclusive NorthAmerican distributor for Mater-Bi, selling products, including bagsand cutlery, under the reSource name.154

EarthShell Corporation, in Santa Barbara, Calif., markets packagingmade by combining limestone, starch, and cellulose fiber, mixing thematerials with water and then baking them in a mold The evaporation

of the water and the resulting steam thermoforms the product.218Containers reportedly dissolve completely in water after they are broken,and are compostable The company also claims the containers use onlyhalf the energy of competing products, and generate fewer pollutants.219

12.6.3 Protein-based plastics

Rather than focusing on starch-based plastics, some investigatorshave worked on developing protein-based plastics from corn or othersources Film made from zein, a protein found in corn gluten mealafter production of ethanol, has been studied at the University ofIllinois at Urbana-Champaign They have produced a biodegradable,water-resistant shrink wrap The first test application was to protecthay bales from wet weather, substituting for plastic covers that must

be removed and disposed of In addition to being biodegradable, thezein-based wrap has nutritional value, and can be eaten by live-stock.220Another application studied was mixing zein with fatty acidsand flax oil to produce water-resistant food containers, plates, sealingmaterial, and trays Untreated zein was found to be brittle and toabsorb too much water The addition of fatty acids and flax oil as plas-ticizers solved that problem, without interfering with the biodegrad-ability of the material The cost in 1997, however, was over $5/lb.221Researchers at the National Food Research Institute in Japan and

at Clemson have also studied zein from corn as a biodegradable

pack-aging material, in film form and as a paper coating Zein films andzein-coated paper were shown to be heat-sealable and compostable, aswell as suitable for animal feed The investigators also claimed thatzein-coated paper is recyclable with ordinary paper The materialcould also be modified with plasticizer.222

R Narayan at Michigan State University has investigated the use

of soy protein to manufacture biodegradable plastic films While earlysoy-protein films were weak and brittle, he has extrusion-blended soyproteins with selected aliphatic polyesters to produce water-resistantbiodegradable films with higher elongation and tensile strength.Potential applications range from plastic cutlery to containers forseedlings Some blends have elongation at break of 500%, with tensilestrengths around 2000 lb/in2.223

Investigators at North Dakota State University have producedfilms from soy isolate plus a plasticizer, and are identifying chemicalagents to produce cross-linking of the proteins through the lysineresidues they contain in an effort to improve tensile strength andelongation.224

Iowa State University had a research project from 1991 to 1994 onusing soy protein to make plastics similar to thermosets, using variousaldehydes and acidic anhydrides as cross-linking agents They alsoinvestigated the use of cellulose fibers as fillers in the plastic, usingstarch as an ingredient, and soy-protein-based foams Closed-cellfoams with densities of about 0.2 g/cm3were successfully produced.225Researchers at Auburn University and the University of Alabama atBirmingham have produced a synthetic gene which induces the for-mation of protein-based polymers in microorganisms, and have suc-cessfully transferred the gene to tobacco cells The plants showedpolymerlike inclusions in the tobacco leaves.226

Showa Highpolymer Company of Tokyo, Japan, has developed abiodegradable thermosetting resin described as an amino proteinresin Its biodegradability is 31% weight loss after 31 weeks in stan-dard soil, reportedly greater than that of natural wood Mechanicalproperties are claimed to be on a par with or superior to those of con-ventional thermoset resins.227

12.6.4 Polysaccharides

A limited amount of research has been directed at polysaccharides.Cellulose and thermoplastics derived from cellulose are members ofthis family, of course, as are starch-based polymers It is well known,

as mentioned earlier, that biodegradation is influenced by the degree

of modification of the basic structure of the cellulose as well as by itscrystallinity In 1990, Japanese researchers were investigating the use

of selected natural polysaccharides to produce biodegradable plasticswith controlled rates of degradation, as well as improved elongation,tear strength, and transparency.228

Hayashibara in Japan has produced Pullulan polysaccharide-basedbiodegradable films Properties were reported to be similar to poly-styrene Aisero Chemical in Japan has produced biodegradable filmsand gels made by reacting chitin, found in the shells of crustaceans,with concentrated bases to form chitosan Films manufactured fromchitosan and cellulosic fiber have been reported to be hydrophilic, butnot water-soluble, and to be good oxygen barriers.222

In 1996, Austrian researchers developed a thermoplastic in whichwaste wood chips, corn, and cellulose-derived resins were combined toproduce a biodegradable and melt-processable wood.230

Researchers in Korea, in 1997, announced the development of abiodegradable plastic made from fibers in genetically engineeredaspen trees Their methodology included the use of chemical catalysts

as well as modification of the genetic content of the tree.231

12.7 Synthetic Biodegradable Polymers

Most synthetic polymers are not biodegradable However, there are afew synthetic polymers which are truly biodegradable Some are water-soluble and become biodegradable once dissolved Others are insoluble

in water The major families are lactic acid–based polymers, lactone, other synthetic polyesters, and polyvinyl alcohol

polycapro-12.7.1 Lactic acid–based polymers

Polymers based on lactic acid are inherently biodegradable They havebeen available on a small scale for a number of years Some absorbablesutures, for example, are made from lactic acid The polymer isdegraded by contact with moisture In 1954, DuPont patented thering-opening polymerization process for lactic acid polymers, followingconversion of lactic acid to a cyclic dimer in the first reaction stage.232However, the cost of production of the monomers, especially if stereo-chemically pure enantiomers were desired, was a significant deterrent

to widespread commercial development of the polymers until fairlyrecently New technologies have lowered costs, fueling extensiveresearch around the world For example, researchers at ArgonneNational Laboratory, in Argonne, Ill., developed a process for forminglactic acid from potato waste which cut the greater than 100-h pro-cessing time down to less than 10 h.233One of the key developments

was the ability to control the ratio and distribution of the d- and l-forms

of lactic acid in the polymer backbone by modifying polymerizationconditions

Lactic acid can be formed by either chemical or biological processes.Fermentation processes provide more ability to control the enan-tiomers being produced Bacteria have been identified which form both

L and D enantiomers Some preferentially form D, others tially form L, and still others form significant amounts of both enan-tiomers.232 The ratio of the two forms in the polymer affectscrystallization kinetics, melting temperature, and polymer rheology.234

preferen-L enantiomers are also known to be present in mammalian systemsand easily assimilated by humans.232

One of the earliest companies to extensively develop polylactic acid(PLA) polymers was Cargill, which began working on them before 1987,and began production of pilot plant quantities in 1992 Cargill marketedbiodegradable lactic acid polymers under the EcoPLA trade name InDecember 1997, Dow Chemical and Cargill, after 15 months of jointinvestigation, formed a joint venture, Cargill Dow Polymers, to furthercommercialize PLA polymers Cargill already had a 4000-metric ton/yearfacility near Minneapolis, and was building a 35,000-metric ton/yearplant in Blair, Neb The joint venture plans to build a 125,000-metricton/year plant by 2001.235,236

The starting material for PLA is starches or sugars from corn,

sug-ar beets, or other sources The stsug-arch is converted into sugsug-ar, andthen fermented to yield lactic acid Next, water is removed to yield alactide Solvent-free polymerization is then used to produce PLA Itsproperties make PLA suitable for a wide range of applications Itsgloss, clarity, temperature stability, and processability are compara-ble to polystyrene, and its grease and oil resistance and odor and fla-vor barrier properties are comparable to PET It is heat sealable at alower temperature than polyethylene and polypropylene PLA can beformulated to be flexible or rigid, and can be copolymerized with oth-

er materials to further modify its properties It is not water-soluble.Other properties are listed in Table 12.4 Stereochemistry can also bevaried to produce amorphous, semicrystalline, or crystalline poly-mers It can be processed by most conventional plastics forming oper-ations, including blown film, thermoforming, extrusion, and injectionmolding The selling price for the resin in 1997 was above $1.00/lb,

but Cargill and Dow expected it to fall to about 50¢/lb by 2001, withincreases in production.235,237 Polylactic acid has Generally Regarded

As Safe (GRAS) status for food packaging.234

Potential applications for PLA include cast, blown, and orientedfilms; rigid containers; and coating for paperboard A major application

is compost bags PLA resins were also being used for consumer aging in Japan, and injection molding applications in both Japan andEurope Other PLA manufacturers include Mitsui Toatsu andShimatsu in Japan, Chronopol in Colorado, and Neste Oy inFinland.235,237Mitsui Toatsu in 1998 introduced an improved genera-tion of its PLA called Lacea Chronopol had a semicommercial resintrade-named Heplon, with a world scale plant planned within the nextfew years.238

pack-Duro Bag Manufacturing Company EcoPLA bags consisted ofthree-layer blown film, with an interior layer of polylactide sand-wiched between layers of a proprietary biodegradable aliphatic poly-ester The bags were intended to replace paper bags for collection ofcompostables.239

Paper drinking cups made with an extrusion coating of PLA havegreater stiffness than polyethylene-coated cups However, unmodifiedPLA does not perform well in extrusion coating because of its low meltelasticity Therefore, for such applications, it is desirable to modify themelt elasticity by modifying the molecular weight distribution anddegree of branching Investigators from Cargill Dow were able toincrease branching by using epoxidized soybean oil as a multifunc-tional comonomer and also by reacting PLA with peroxide They foundthe peroxide reaction to be the most favorable method for inducing theformation of a highly branched molecular structure which was suitablefor extrusion coating of paper.234

CornCard International manufactures a degradable polylactic based polymer called Mazin It also contains a proprietary additivepolymer The lactic acid is derived from corn The polymer has beentested at the University of Nebraska to verify its biodegradability.240Dainippon Ink and Chemicals, Inc (DIC), of Japan, is developingbiodegradable plastics using lactic acid copolymerized with a

acid-TABLE 12.4 Properties of Pure Poly(L-Lactide) and Dainippon’s CPLA 232,241

Tensile Flexural Melting point, strength, modulus, O2 permeability,

*Tensile modulus.

biodegradable aliphatic polyester that is termed CPLA The aliphaticpolyester is formed from dicarboxylic acid, glycol, and other sub-stances CPLA is reported to break down into a low molecular weightpolymer by hydrolysis after 3 to 6 months of exposure in soil, sludge,

or seawater and to begin decomposing in 6 to 12 months by microbialaction If composted with food garbage, it begins to break down inabout 2 weeks The materials can be transparent and flexible or rigid,depending on the amount of the polyester The soft flexible materialcontains a substantial amount of polyester, on the order of “a few tenspercent.” The hard rigid material has only a small percentage of poly-ester Both materials have a density of about 1.25 g/cm3, and a meltingpoint at just over 160°C The glass transition temperature for the hardmaterial is 60°C, and 51°C for the soft material The resins can bemolded by extrusion or injection, including blow-molding, and can also

be used to produce foam or spun to produce fiber They are said to bestable at temperatures up to 200°C, and to be easily vacuum formed,heat sealed, and printed Other properties are shown in Table 12.4.241Many other copolymers can also be created from lactide or lactic acidand other appropriate monomers Crystalline copolymers of L-lactideand glycolide are used as medical sutures Copolyesters of lactide andglycolide are sold under the name Vicryl and have a melting point of210°C and a glass transition temperature of 43°C Amorphous copoly-mers of L-lactide and D,L-lactide with glycolide have been suggestedfor biodegradable drug delivery systems Block copolymers of L-lactideand -caprolactone may also be suited for biomedical uses

As mentioned, the enantiomeric purity of lactic acid polymers nificantly affects their properties Pure poly (D-lactide) and pure poly(L-lactide) are crystalline materials Poly (L-lactide) typically hasabout 40% crystallinity Poly (D,L-lactide) is totally amorphous.Copolymers of D-lactide or L-lactide with D,L-lactide may or may not

sig-be crystalline, depending on the amount of comonomer Blends of poly(L-lactide) and poly (D-lactide) are reported to have better mechanicalproperties than either of the homopolymers alone.232

Unmodified lactic acid polymers tend to degrade rather slowly Oneapplication which makes use of this fact is fishing line Such lines willcompletely decompose after 1 y in seawater Biodegradable shrinkfilms have also been produced by melt-mixing of PLA with ethylenevinyl alcohol copolymers Foams can also be manufactured.232

12.7.2 Polycaprolactone

Polycaprolactone is a member of the polyester family which has beenused in relatively small quantities for a long time The most well-known supplier is Union Carbide of Danbury, Conn., which sells

polycaprolactone resins under the brand name Tone These polymersare compostable but not water soluble.216A primary application is forcompostable bags for yard waste or other organics collection Theycan be utilized as homopolymers, copolymers, or in blends Forexample, R Narayan, of Michigan State University, has investigat-

ed Envar, formed from polycaprolactone and thermoplastic starch,

as a substrate for biodegradable compost bags.215 Other companiesusing starch/polycaprolactone blends are discussed in Sec 12.5.2

12.7.3 Other polyesters

DuPont has created a family of synthetic polyesters which are

degrad-ed by a combination of hydrolysis and microbial action These als, sold under the Biomax name, are similar to polyethyleneterephthalate, but incorporate as many as three different proprietaryaliphatic monomers into the structure The monomers create weakspots which are susceptible to hydrolysis The much smaller moleculeswhich result are biodegradable DuPont claims that in compostingoperations the materials are totally harmless and cannot be undetect-

materi-ed by the unaidmateri-ed eye after about 8 weeks The resins can be tured with existing equipment and existing bulk monomers, so they areonly marginally more expensive than PET Applications include house-hold wipes, yard waste bags, components in disposable diapers, dispos-able eating utensils, geotextiles, agricultural films, plant pots, coatedpaper products, adhesives, and more Biomax can be thermoformed,blow molded, and injection molded Properties can be customized tomeet the demands of the application Melting points are around 200°C,which is significantly higher than in many biodegradable materials.Strength can be formulated to be as low as LDPE or as high as half thestrength of polyester film Elongation can range from 50 to 500%.242Eastman, in 1998, announced the development of Eastar Bio copoly-ester 14766 The patented polymer is derived from conventionaldiacids and glycols but is completely biodegradable and compostable.Projected markets include lawn and garden bags, food packaging, andhorticultural applications Properties are similar to low-density poly-ethylene, and it can be processed on conventional equipment to makeblown or cast film, extrusion coated, or woven into fiber and netting It

manufac-is water-resmanufac-istant, imparts no odor or taste to food or beverages, andhas no plasticizers or other migratory substances The compost time iscomparable to newspaper, that is, 60 to 90 days The material meetsstandards for a variety of food-contact applications It is being target-

ed to areas where composting is common The polymer can be usedalone, blended with starch or wood flour, or as a coating on paper Cost

is reported to be low, and the material is recyclable.243

Other companies manufacturing biodegradable synthetic aliphaticpolyesters include Showa Highpolymer of Japan, which has manufac-tured Bionolle biodegradable synthetic polyester for several years.Bionolle can be processed on conventional polyolefin equipment at 160

to 260°C, and can be blown, extruded, or injection molded It is sealable, and has better resistance to water and organic solvents thanmany other biodegradable plastics Bionolle has good environmentalstress crack resistance, higher yield strength than polyethylene, andstiffness between HDPE and LDPE Izod impact strength was reported

heat-to be adequate for most end uses In compost conditions, bottles appeared from view after 4 weeks Films buried in moist soil nearlydisappeared after 1 year At least four types of Bionolle have been pro-duced, with somewhat different properties, including degradationrates.244Some of the materials are extended with diisocyanate.245Bayer, in 1995, produced a synthetic polyester amide material which

dis-is biodegradable It dis-is reported to have a melt temperature of 125°C,

to have good resistance to solvents and light, and to be strong andextremely tough Its biodegradability meets the German DIN stan-dard It can be extruded into films, injection molded, blow molded, orspun into fibers Properties are similar to low-density polyethylene.The material is described as crystalline and translucent.246 Bayerbegan marketing BAK 1095 in the United States, Europe, and Japan

in September 1997, and was developing BAK 2195, which was able only in sample quantities.154In 1998, Bayer announced the devel-opment of a new biodegradable plastic, which is recyclable as well asbiodegradable Tests on 200-m film showed biodegradability in under

avail-70 days in compost conditions Compostability reportedly is not affected

by multiple recycling The chemical makeup of the resin was notdescribed.247In early 1999, the company lists biodegradable polyesteramide only as a developmental product, with BAK as its provisionaldesignation.248

In 1995, BASF announced the development of a biodegradable moplastic copolyester based on starch feedstock A plasticizer report-edly is blended on-line with the copolyester to form a material which

ther-is antther-istatic, has good toughness, and good elongation at break.249 In

1999, BASF listed Ecoflex as the trade name for their biodegradablepolyesters.250

12.8 Water-Soluble Polymers

Several water-soluble polymers are known which are stable in thesolid state but will biodegrade once they are dissolved These includeboth synthetic and natural polymers, such as polyvinyl alcohol, cel-lulose esters and ethers, acrylic acid polymers, polyacrylamides, and

polyethylene glycol, along with natural polymers derived from starchand some polylactides Polyethylene oxide is biodegradable at molec-ular weights below 500.251For many of these materials, the degree ofwater solubility can be altered by changes in the polymer formula-tion Several of these polymers have already been discussed Othersare discussed here.

12.8.1 Polyvinyl alcohol

Among the first manufacturers of polyvinyl alcohol (PVOH) were AirProducts & Chemicals of Allentown, Pa., and ChrisCraft IndustrialProducts, Inc., of Gary, Ind Currently, Air Products, which sells polyvinylalcohol under the Airvol trade name, targets its resins primarily at adhe-sives and paper coatings ChrisCraft sells PVOH film under the MonoSoltrade name.251-253 Nippon Gohsei claims to have the second largestpolyvinyl alcohol capacity in the world at 6000 tons/month Its resins areused in textiles, paper processing, emulsifiers, sizing, and adhesives aswell as for biodegradable materials.254Italway, of Italy, sells polyvinylalcohol polymers under the name Hydrolene.255

Polyvinyl alcohol is produced by hydrolysis of polyvinyl acetate,since the vinyl alcohol monomer is unstable The degree of solubilitycan be controlled by modifying the extent of hydrolysis and the molec-ular weight of the polymer Films can be produced which are readilysoluble in water of any temperature or soluble in hot water only.Applications include hospital laundry bags and soluble pouches forchemicals and detergents Most applications require the pouch to becontained in an outer barrier package to prevent moisture from reach-ing the material prematurely

One difficulty in using this polymer is that it is generally not meltprocessable, since its decomposition temperature is lower than itsmelting point Films are typically prepared by a solution castingprocess from a water solution Plasticizers are incorporated in someresins The films are readily biodegraded in wastewater streams or incompost It is printable using either water-based or solvent-basedinks The materials are resistant to most organic liquids, including sol-vents, and to mineral oils Film can be used to package products hav-ing low water contents but not high ones.252

12.8.2 Polyoxyethylene

Polyoxyethylene is a water-soluble polymer which has been known for anumber of years but has seen little use Reports about its biodegradabil-ity disagree Some experts claim the polymer is inherently biodegrad-able, while others state it only biodegrades at molecular weights under

500 Polyoxyethylene was available in the United States from PlanetPolymer Technologies through Mitsubishi in 1993.256

12.9 Summary

Recycling of plastics continues to grow around the world, although ithas had some setbacks in the United States Financial support fromthe plastics industry has declined in the United States, making it cru-cial that recycling programs be economically viable to survive.Developments in further mechanization of plastics sorting can loweroperating costs, but sometimes require a financial investment that isout of reach Regulatory pressure for recycling of all types hasdecreased in the United States in the last several years, but there isthe potential for it to rapidly increase again if the public perceives asignificant decline in the availability of recycling In several otherareas of the world, including the European Union, both recycling ratesand regulatory pressure continue to increase

The philosophy of producer responsibility seems to have gained astrong position in Europe, and is making substantial impact in Canada,Asia, and Latin America The first rumblings of this movement havealso been heard in the United States, primarily at the state level Thereare serious consequences for industry in states continuing to take thelead in waste-related areas In addition to having to make its case in 50places rather than one, industry always runs the risk that one statemay prohibit exactly the thing that another state requires

Use of life-cycle assessment techniques to analyze material choices,processes, and waste disposal continues to increase Some countries inEurope require life-cycle analysis before products are introduced TheU.S EPA and the Department of Energy have jointly sponsoredresearch to develop the tools and information needed for life-cycleanalysis-based decisions about solid waste management strategies.The results of this project have already undergone peer review byexperts, and are scheduled to be released in 2000 This study includesboth economic and environmental aspects, and will have relevanceinternationally as well as in the United States.257

On a related note, concerns about greenhouse warming may lead tosignificantly higher prices for fossil fuel or even to limitations on energyuse To the extent that the plastics industry can take steps ahead oftime to modify operations in ways that make them more energy effi-cient, the industry will be better prepared for whatever action results.Further, if it is determined that no action need be taken, the industrywill still reap the economic benefits of lower fuel expenditures

As composting of source-separated organics becomes more common,the potential markets for biodegradable plastics are likely to increase

How much actual markets increase will be a function of cost To theextent that biodegradable plastics become cost competitive with com-modity plastics, their use will grow If their prices do not decrease sig-nificantly, they are likely to continue to have a role only in minor nichemarkets Of course, regulatory pressure could change this as well.

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